The present invention generally relates to hand held communication devices employing whip antennas. A whip antenna is an antenna having a single straight flexible wire or rod. The bottom end of the whip is connected to the radio receiver or transmitter.
For hand-held long range communications, the band is typically in the range of 2-30 MHz. The shorter frequencies have the ability to follow the contours of Earth. This is one of the few benefits over high frequency communications, which may be more limited to line of sight. Unfortunately, as frequency reduces, the whip length should increase to maintain efficiency.
Some conventional hand-held whip antenna communication devices that operate in the 90-500 MHz band have a whip antenna of lengths of about four feet. Such a length is not very practical for a hand-held device. A tri-fold version provides a collapsible antenna having a much shorter length when not in use. However the folded antenna must be deployed to the full 4 ft length for use. Another type of conventional hand-held whip antenna communication device uses a twelve inch whip antenna. This conventional “short” whip antenna employs a transformer to reduce impedance mismatch between the signal generator and the antenna. This will be described in reference to FIG. 1.
FIG. 1 illustrates a short whip antenna transmission system 100.
As shown in the figure, transmission system 100 includes a signal generator 102, a transformer 104 and an antenna 106. Signal generator 102 is connected to transformer 104 at a node 108 and transformer 104 is connected to antenna 106 at a node 110.
Signal generator 102 generates an alternating current signal for use by antenna 106 to transmit a corresponding radiated signal. Transformer 104 reduces an impedance mismatch between signal generator 102 and antenna 106. Antenna 106 is a short whip antenna for transmitting in the 90-500 MHz range.
In this example, the output impedance of signal generator 102, at node 110, is 50 Ω and the input impedance of antenna, at node 110, is 300 Ω. Such an impedance mismatch would drastically reduce the efficiency of transmission system 100. Tremendous heat is generated by transformer 104. As a result a heat sink is used to transfer and dissipate heat to the environment. This will be described with reference to FIG. 2.
FIG. 2 illustrates a conventional short antenna 200 for transmitting at least 90 MHz.
As shown in the figure, conventional short antenna 200 includes a connector 202, a circuit board 204, a short whip antenna portion 206 and a heat sink 208. Circuit board 204 includes a toroidal transformer 210.
Connector 202 is connected to circuit board 204, which is additionally connected to short whip antenna portion 206. Heat sink 208 is thermally connected to toroidal transformer 210.
Connector 202 receives a signal from a signal generator (not shown) and conducts the signal to circuit board 204. Consider the situation where the signal generator has an output impedance of 50 Ω and short whip antenna portion 206 has an input impedance of 300 Ω. Just as discussed above with reference to FIG. 1, in this case, toroidal transformer 210 provides an impedance matching function. However, toroidal transformer 210 generates heat, which is dissipated via heat sink 208.
FIG. 3 illustrates a cross-sectional view of heat sink 208 along plane X-X of FIG. 2.
As shown in FIG. 3, heat sink 208 includes a tubular body 302 and a plurality of heat fins, a sample of which is numbered 304. Tubular body has a hollow center 306.
Returning to FIG. 2, as heat is generated by toroidal transformer 210, the heat is conducted to tubular body 302 of heat sink 208. Tubular body 302 then conducts the heat through its fins, for dissipation to the environment.
Connector 202 may be a standard coaxial connector. Heat sink 208 is manufactured to fit connector 202 and connect to standard short whip antennas, such as short whip antenna portion 206. The combined function of the impedance matching of toroidal transformer 210 with the heat dissipating function of heat sink 208 enables to somewhat efficient short whip antenna hand held communication device operable at lower frequencies. This will be described with reference to FIG. 4.
FIG. 4 illustrates a graph 400 of VSWR as a function of frequency of the driving signal.
As shown in the figure, graph 400 includes a Y-axis 402, an X-axis 404, a function 406, a function 408, a function 410, and a dotted line 412.
Y-axis 402 is a voltage standing wave ratio (VSWR). A standing wave ratio (SWR) is a measure of impedance matching of loads to the characteristic impedance of a transmission line. The SWR may be thought of in terms of the maximum and minimum AC voltages along the transmission line, thus being called the VSWR. In graph 400, Y-axis 402 is the VSWR and is measured logarithmically. It is a goal to reduce the VSWR as much as possible for the band with which a transmitter will be transmitting. In other words, with the respect to VSWR, the lower—the better.
X-axis 404 is frequency in MHz of the transmitted signal.
Function 406 corresponds to the VSWR as a function of frequency of to transmission system having a four foot long whip antenna. Function 408 corresponds to the VSWR as a function of frequency of a transmission system having a four foot long tri-fold whip antenna. Function 410 corresponds to the VSWR as a function of frequency of a transmission system having a short whip antenna as illustrated in FIG. 2.
Dotted line 412 represents a VSWR threshold for a particular transmitter requirement. In this example, dotted line 412 highlights a VSWR value of 3.
As shown in the graph, function 406 has a VSWR value below 3 from about 80-120 MHz, whereas function 408 has a VSWR value below 3 from about 80-105 MHz. Function 410 has a VSWR value below 3 at greater than about 90 MHz.
What is needed is a short whip antenna that can provide a VSWR value below 3 at less than 90 MHz and that can fit within a conventional heat sink as shown in FIG. 2.